The invention relates to a lighting unit comprising a supply unit and a low-pressure mercury discharge lamp, which low-pressure mercury discharge lamp has a light-transmitting discharge vessel which is provided with a luminescent layer on an inner surface and which encloses a discharge space in a gastight manner, the discharge space being provided with a filling comprising one or several rare gases in addition to mercury. In this discharge space a first and a second electrode are positioned, which electrodes each comprise a coiling of a metal wire coated with one or several metal oxides which emit electrons, and which electrodes are each electrically connected to a respective current supply conductor which extends to outside the discharge vessel and is electrically connected there to the supply unit, which unit ignites the low-pressure mercury discharge lamp in the cold state upon switching-on.
Lighting units with a low-pressure mercury discharge lamp, also referred to as "lamp" hereinafter, are widely used for general lighting purposes. They render possible a considerable energy saving owing to the high luminous efficacy in comparison with that of incandescent lamps.
A discharge is maintained with a discharge current between the electrodes during nominal operation of the lighting unit. The manner in which the lamp is ignited depends on the type of lighting unit.
In a lighting unit of the "hot start" type, the electrodes each have a further current supply conductor. Before an ignition voltage is applied between the electrodes, the latter are brought to a temperature required for nominal operation by means of an auxiliary current from the current supply conductor to the further current supply conductor. This method of igniting, however, leads to a comparatively long delay, called ignition delay hereinafter, between the moment of switching-on of the lighting unit and actual lamp ignition. This is a disadvantage. Moreover, the provisions required for this render such a lighting unit comparatively expensive.
In a lighting unit of the "rapid start" type, the electrodes also each have a further current supply conductor, and the electrodes are given a raised temperature by means of an auxiliary current. It is true that an ignition voltage is applied immediately after switching on in this type of lighting unit. An ignition delay is realized here, however, in that the ignition voltage is insufficiently high for igniting the lamp with its still cold electrodes. The lamp does not ignite until after the electrodes have assumed a sufficiently high temperature.
A lighting unit of the kind described in the opening paragraph, in which the lamp ignites in the cold state, is described in U.S. Pat. No. 5,341,067. In such a lighting unit, also referred to as "cold start" or "instant start", an ignition voltage is applied between the electrodes of a sufficient level for igniting the lamp practically immediately after switching-on, i.e. with an ignition delay of less than 100 ms. The electrodes only assume their nominal operational temperature after ignition. A further current supply conductor to the electrodes is not necessary in lighting units of this type. The electrodes are exclusively heated by the discharge arc applied thereto and by the discharge current during nominal operation in this case. If the electrodes have a further current supply conductor, an auxiliary current may contribute to maintaining the nominal operational temperature of the electrodes.
The lamps of the lighting units mentioned above in general have a long life in continuous operation. The life of lighting units in which the lamps are ignited in the cold state is limited in practice by the switching life of the lamp, i.e. the number of times the lamp can be switched on. It was in fact found that the ignition of the lamp whose electrodes are still cold causes comparatively much damage to the electrodes, so that the electrode becomes defective after a comparatively small number of switching operations. To reduce the switching frequency, the lamps are often operated continuously also in cases where the need for lighting is of short duration only. This strongly detracts from the energy saving which could be achieved in comparison with incandescent lamps.